A variety of materials have been blended with soils to enhance or improve the properties thereof. In early highway construction, soil and rocks were mixed to provide a more stable, free draining, better performing roadbed. Lime has routinely been added to clay and silty soils to reduce their plasticities and to reduce their swelling potential. Portland cement has been added to several types of soils, being mixed in place or in a batch plant for achieving an improved highway base material. More recently, woven synthetic materials have been placed in horizontal layers of soil in order to achieve steep, stable earth slopes.
Examples of the last technique, involving the use of so-called geotextiles, have been described in the patent literature. U.S. Pat. No. 3,934,421, for instance, is directed toward a matting of continuous thermoplastic filaments that are bonded together at intersections. When placed in loose soil, the matting provides increased vertical load bearing capacity and resistance to lateral deformation.
U.S. Pat. No. 4,002,034 also discloses a matting, anchored to the ground, for preventing erosion. The matting is a multi-layered composite providing an uppermost layer having the finest fibers and least pore spaces and a ground side layer having the thickest fibers and greatest pore spaces.
U.S. Pat. No. 4,329,392 provides a layered matting designed to inhibit rearrangement of soil particles. The matting comprises melt-spun synthetic polymer filaments with macrofibers forming a web, a filter layer of finer fibers bonded thereto and an intermediate layer of other fibers therebetween. The mat has use below water level to control erosion.
U.S. Pat. No. 4,421,439 is directed toward woven fabric, comprising filaments such as polyester, polyamides and polyolefins. The fabric is positioned beneath sand, gravel, stones, clay, loam and the like at a depth of at least 10 cm. The invention is based on the particular construction of the fabric which gives it improved load bearing performance.
Another unique configuration of geotextile material is disclosed in U.S. Pat. No. 4,472,086. The material is used as a reinforcement for the construction of roadways and on slopes and river banks to control erosion.
Despite the wide-spread use of polymer filaments in fabrics, matting and the like as a reinforcement for soil, the foregoing patents have not taught the use of individual fibers blended with the soil. Discrete fibers have been employed heretofore in the reinforcement of concrete as set forth in U.S. Pat. No. 3,645,961. The patent discloses the use of nylon, polyvinyl chloride and simple polyolefins in lengths ranging between one-quarter to three inches (0.4 to 7.5 cm) to form a blast resistant concrete.
Actually, polypropylene fibers have been used to modify the behavior of concrete for over 20 years. Improvement in water tightness, reduction in cracks, toughness, ductility, and impact resistance have been noted. Steel fibers have also been used for this purpose with limited success. Nevertheless, few studies on fiber reinforced soil have been reported. Those that exist have generally centered around attempts to understand the effects of roots of vegetation on embankment slope stability, particularly of earth dams. Thus, improving the engineering properties of soil in this manner appears not to have been investigated heretofore.